1. Field of the Invention
The present invention relates to a group III nitride semiconductor device (hereafter also referred to as a device simply) and, particularly to a fabrication method of a semiconductor laser device using the same material system.
2. Description of the Related Art
A semiconductor laser device has a resonator consisting of a pair of flat parallel planes or reflectors between which a multilayer semiconductor laser structure is formed. To operate the laser device, a pair of the mirror facets defining an optical cavity is necessary. For example, in case of a GaAs semiconductor laser device, since an epitaxially grown GaAs crystal used for the device and a GaAs wafer for the substrate thereof have a cleavage nature, this nature is utilized for the fabrication of the device. The resonator is formed in such a manner that straight lines of grooves are carved to the GaAs wafer at a predetermined interval of cavity length and thus, the wafer is cleaved along the grooves into bars by application of stress. Therefore, the cleavage of wafer facilitates to form automatically flat mirror planes parallel to each other in a process for forming the laser structure.
Thus, in the fabrication of a semiconductor laser device of Fabry-Perot type using a conventional semiconductor crystal such as GaAs, the mirror facets of the device have been formed by using the same cleavage properties of GaAs crystal substrate and GaAs multilayers.
On the other hand, in the case of group III nitride semiconductor device, it is inevitable to perform the epitaxial-growth of nitride crystal film on a substrate made of sapphire or SiC, because a nitride bulk crystal to be used in practice has not been manufactured yet.
SiC is not frequently used as a substrate for the nitride device, because the SiC substrate is expensive and a nitride film deposited on the SiC substrate easily cracks due to the difference in the thermal expansion coefficient therebetween, thus sapphire is commonly used as a substrate. In the case of epitaxial growth of nitride on a sapphire substrate, a high quality single-crystal film is obtained on a C-face i.e., (0001) plane of sapphire, or an A-face, i.e., (11{overscore (2)}0) plane (hereafter referred to as (11-20) plane) of sapphire.
The mirror facets may be formed by an etching process such as reactive ion etching (RIE) instead of the method using the cleavage, because it is hard to crack the sapphire substrate in comparison with the GaAs substrate having been used so far for semiconductor laser devices.
Reactive ion etching is mainly used as a method for obtaining the mirror facets of the nitride semiconductor laser on the sapphire substrate at present.
However, the resultant device with the mirror facet formed by the reactive ion etching has a disadvantage that the far-field pattern of its emitted light exhibits multiple spots. This multiple-spot phenomenon of the laser device is caused by the fact that the sapphire substrate cannot be effectively etched even by dry etching such as the reactive ion etching.
FIG. 1 shows a cross section of a laser device 1 fabricated on a sapphire substrate 3 with a mirror facet 2 formed by an etching process. A remaining portion of the sapphire substrate 3 without being etched as shown by (A) in FIG. 1 reflects a part of emitted light beam, an then the reflected light interferes with the main light beam, so that a far field changes to multiple spots. Since the alteration of the far-field pattern into multiple spots is fatal as a light source for reading an optical disk, making the device impractical.
A GaN laser have been initially fabricated by using an etched mirror obtained by the reactive ion etching. The mass-production-type GaN laser with the cleaved mirror is studied in view of the change of a far field pattern to multiple spots. It is a matter of course that a cleavage cannot be preferably formed on sapphire in mass production. Therefore, the following method have been used. First, after forming a GaN film with a thickness of approximately 2 xcexcm on a sapphire substrate by metalorganic chemical vapor deposition (MOCVD), the substrate with the film is temporarily taken out of a reactor. A SiO2 film is formed on the GaN film and stripe-like windows are opened on the film. After putting the film in the MOCVD system again, a GaN film is grown up to a thickness of approximately 10 xcexcm to obtain a flat film. After that, the obtained wafer is subject to the hydride vapor phase epitaxial processes (HVPE) to form a GaN film on the wafer up to a thickness of approximately 200 xcexcm. Next, the backside of the sapphire substrate of the obtained wafer is lapped to remove almost of the sapphire portion, and then the GaN substrate having a thickness of approximately 80 xcexcm is obtained. The resultant substrate is set in the MOCVD system too perform epitaxial growth of a laster structure. Because the obtained wafer is very similar to a wafer of a conventional GaAs-based laser, it is possible to apply various treatments and thereafter cleaving the crystal substrate. In this way, a laser device is fabricated.
However, the conventional method as described above requires many steps, and is complicated. As a result, the method invites a very low yield of the group III nitride semiconductor devices. Such a method is not suitable for mass production.
Therefore, an object of the present invention is to provide a group III nitride-semiconductor laser fabrication method from which high-quality mirror facets for a laser structure can be obtained with high reproducibility.
A nitride semiconductor laser device according to the present invention having successively grown crystal layers each made of a group III nitride semiconductor (AlxGa1xe2x88x92x)1xe2x88x92YInyN (0xe2x89xa6xxe2x89xa61, 0xe2x89xa6yxe2x89xa61) comprises:
a cleavable or parting substrate;
a crystal layer made of the group III nitride semiconductor over the substrate;
a mirror facet for optical resonance consisting of a cleavage plane of the group III nitride semiconductor; and
a decomposed-matter area of the nitride semiconductor interposed at an interface between the substrate and the crystal layer and disposed at an intersecting portion with the cleavage plane.
In an embodiment of the nitride semiconductor laser device according to the invention, the decomposed-matter area of the nitride semiconductor is formed by a light beam applied to the interface from the substrate side.
In another embodiment of the nitride semiconductor laser device according to the invention, the device further comprises a waveguide extending along a direction normal to the cleavage plane of the nitride semiconductor.
In a further embodiment of the nitride semiconductor laser device according to the invention, the waveguide has a ridge shape.
In a still further embodiment of the nitride semiconductor laser device according to the invention, the substrate is made of sapphire.
A fabrication method according to the present invention is a method for producing a nitride semiconductor laser device having crystal layers each made of a group III nitride semiconductor (AlxGa1xe2x88x92x)1xe2x88x92YInyN (0xe2x89xa6xxe2x89xa61, 0xe2x89xa6yxe2x89xa61) and layered on a cleavable or parting substrate in order, the method comprising the steps of:
forming a plurality of crystal layers each made of a group III nitride semiconductor (AlxGa1xe2x88x92x)1xe2x88x92YInyN (0xe2x89xa6xxe2x89xa61, 0xe2x89xa6yxe2x89xa61) on a cleavable or parting substrate;
applying a light beam from the substrate side toward the interface between the substrate and the crystal layer thereby forming the decomposed-matter area of a nitride semiconductor; and
cleaving the substrate along a straight line intersecting the decomposed-matter area, thereby forming a cleavage plane of the crystal layer.
In an embodiment of the fabrication method according to the invention, the wavelength of the light beam is selected from wavelengths passing through the substrate and absorbed by the crystal layer in the vicinity of the interface.
In another embodiment of the fabrication method according to the invention, the method further comprises a step of forming a waveguide extending along a direction normal to the cleavage plane of the nitride semiconductor.
In a further embodiment of the fabrication method according to the invention, the crystal layers of the nitride semiconductor are formed by metal-organic chemical vapor deposition.
In a still further embodiment of the fabrication method according to the invention, the method further comprising a step of applying a light beam focused on an outer surface of the substrate to form a groove serving as a starting point of the aforementioned straight line in the step of forming the mirror facet for optical resonance.
According to the present invention, it is possible to obtain high-quality mirror facets with a high reproducibility by locally untying the crystal bond between the sapphire substrate and GaN crystal nearby the mirror facets of a laser device and thereby, cutting the GaN crystal nearby the laser facet cleave along its normal cleavage plane.